1. Field of the Invention
The present invention relates to a method and a device for limiting the current of a cathode-ray tube.
2. Description of the Prior Art
Television receivers are fitted out with an average beam current limiter (or ABCL) system making it possible to obtain sufficiently luminous pictures (in terms of contrast and light), inasmuch as the current remains below a certain limit (for example 1.2 mA). The ABCL further makes it possible to reduce the content of the picture (generally the contrast and/or the light) when the image that would be obtained without this system implies a beam current which, in the long term, would prompt a premature ageing of the cathode-ray tube.
To this end, the beam current is determined, for example either by detecting the mean current at the base of the EHV (extra-high voltage) source or by measuring the sum of the instantaneous currents of each cathode and by integrating it.
The time constant in every case should be:
short enough so that a sudden transition in the picture content does not prompt an excessive current (some mA) for an excessively long time (for the duration of some frames) in such a way that the EHV source can give the corresponding energy (and also so that the tube is not subjected to excessively high light intensities); PA1 long enough for the light and contrast (or even saturation) levels to remain practically unchanged during a frame. This is particularly difficult to achieve when the upper half of an image is white (or black) and the lower half is black (or white respectively).